It is very difficult question because traders’ experiences vary greatly. Two perfect traders can find the same broker good and bad because the broker handled their orders in different market conditions - liquidity, spreads, day time, other underlying economic and political factors affecting assets on the market. General advice is to determine broker performance in the long run, estimating ratio of where it is behaved good and bad. This ratio should be high, only 10% bad cases is acceptable. Personally I find Tickmill, Hotforex, IB great brokers but their verification can be tough for some types of traders.